If you are talking about a diffuser....
youdirtyfox is right. You can't just throw some crap together and expect it to work. A proper engineered diffuser (one that will actually work) is worth a lot of money.
But, with basic knowledge, you could be successful making your own.
Watch all of these related videos:
http://web.mit.edu/hml/ncfmf.html
Start with modeling your car. Just get the overall shape. Dimensions and large features only, (no need to worry about body seams)
Design the approximate shape around the mounting points you have. It should look like a bent up piece of plywood with vertical fins. There should be an inflection point in the curve.
Run a flow simulation on it. The effectiveness is limited because you cannot specify rotating tires or a moving ground plane (in the software I know of) but you can watch if your flow obviously separates from your curvature. (solidworks has a decent sim, ansys kicks a-- but your computer probably can't run it)
Iterate.
Once you get something that looks good in a sim, go buy 10 odd sheets of that blue polystyrene foamboard.
Glue them all together so they are in the rough shape of your diffuser. Cut it out, likely by hand unless you have a big a-- CNC mill. Apply body filler to make sure its smooth and straight.
Now you're going to have to source a lot of FG (or CF if you are extremely lucky, talk to wyt, he's probably looking) and lay her up. You probably don't have access to an autoclave so the epoxy you are forced use will cause it to have shitty properties. You also need a release agent (wood glue works for low temp cures)
The other option is just modeling it, and then making it out of sheet aluminum. You can specify your model to be made of gauge ally, but make sure you check its weight properties before you proceed. If it weighs more than the downforce you create, redesign.
Have fun. In the end, you'll still wind up with a shitty product. No offence, you just don't have access to the tools you need.
